1. Field of the Invention
The present invention relates to a receiving apparatus and a hybrid automatic repeat request (ARQ) communication system in which data is received using automatic retransmission control and an error correction code to improve a quality of data transmission.
2. Description of the Related Art
In a communication system in which information data is communicated between a transmitter and a receiver, technologies, such as automatic repeat request (ARQ) control and an error correction code (forward error correction (FEC)), are applied to improve data quality of the information data. For example, in a communication system in which processes of retransmitting and combining data are performed, a transmitter processes information bits by adding a parity bit for error detection to a block of the information bits and by performing error-correction-coding, and then transmits all or a part of the information bits processed. At a receiver, when a block of the information bits is received for the first time, the receiver performs error-correction-decoding on the block, and then performs error detection. When no error is detected, confirmation information of positive acknowledgement (ACK) is returned to the transmitter, and when an error is detected, confirmation information of negative acknowledgement (NACK) is returned to the transmitter.
When the transmitter receives the notification information and determines that the notification information is ACK, the transmitter proceeds to transmission processes described above on next block of the information bits. On the other hand, when the transmitter determines that the notification information is NACK, the transmitter retransmits all or a part of coded bits of the block currently being processed. The receiver performs a combining process on the data retransmitted for each bit that corresponds to existing data, and performs error correction and error detection processes again using combined data. In this manner, the receiver repeats, within a predetermined maximum number of times, a trial of the decoding process by notification and retransmission to the transmitter described above until no error is detected in a block.
For the combining process described above, there are known technologies as described below. In a technology disclosed in IEEE Trans. on Common, Vol. 33, pp. 385-393, May 1985 titled “Code Combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets” by D. Chase, for an ARQ scheme in which transmission of a single packet is repeated until all data blocks become error-free, reliability is defined for each received packet, and the reliability is taken as a weight by which likelihood data of each received packet is multiplied. A technology disclosed in Japanese Patent Application Laid-Open Publication No. 2000-4196 relates to a retransmission packet capture system under a wireless multi-service communication environment using turbo combining. In the technology, rake combining is performed on delay pulses of different timings in a wideband code division multiple access (W-CDMA) or the like, based on the technology disclosed in IEEE Trans. on Common, Vol. 33, pp. 385-393, May 1985 mentioned above. In addition, in definitions disclosed in third-generation cellular phone standard, release 5, “3GPP Rel.5”, the Internet information of a value measure for a signal-to-interference ratio (SIR) that indicates a noise state among reception characteristics for a channel of a channel quality indicator (CQI), is added to feedback information of ACK or NACK, and is then transmitted from a receiving side to a transmitting side.
However, in the conventional technologies described above, a process of combining retransmission data is performed at the receiving side, thereby disadvantageously requiring a superfluous decoding process, causing data deterioration as a result of combining and increased power consumption because the combining process and other processes are performed.
First, in the technology disclosed in IEEE Trans. on Common, Vol. 33, pp. 385-393, May 1985, it is defined that “if packet characteristics are deteriorated, nothing is added at all (the weight is 0) in some cases”. In a specific example of such a scheme of determining the weight, when a binary symmetric channel is subjected to maximum likelihood decoding and an error rate Pi of a packet i is known, a weight Wi is defined by
                              w          i                =                  1          ⁢          n          ⁢                                    1              ⁢                              p                i                                                    p              i                                                          (        1        )            and, as for a Gaussian noise propagation path, a dispersion σi2 is used to define the weight as
                              w          i                =                  1                      σ            i            2                                              (        2        )            
If these weights shown above are used as the reliabilities, the combining process is performed every time. Only under an extremely rare condition, the weight accidentally becomes 0 and the combining process is not performed. Therefore, the combining process is performed even when no effect is produced by the combining process. As a result, an unnecessary process is performed by an error rate determining unit, thereby increasing power consumption.
Moreover, in the technology disclosed in IEEE Trans. on Common, Vol. 33, pp. 385-393, May 1985, the combining process is performed every time. Therefore, if data combined includes deteriorated data, this can affect combination result by causing further deterioration to make a state of the data worse than that before combining. Moreover, even when retransmission data is deteriorated and evidently does not provide any improvement, such retransmission data is combined anyway for error correction. Such an unnecessary process increases the power consumption.
Furthermore, in the technology disclosed in third-generation cellular phone standard, release 5, “3GPP Rel.5”, the feedback information to be transmitted to the transmitting side only includes the SIR measurement value. Therefore, it is impossible to grasp, at the transmitting side, the combination process performed at the receiver side and a state of data currently stored. As a result, an appropriate retransmission process cannot be performed.
In view of the above problems, it is an object of the present invention to provide a receiving apparatus that can prevent data deterioration and that reduces the number of retransmission by performing an optimum retransmission requesting process depending on a state of data reception and an error-correction-coding process. Another object of the present invention is to provide a hybrid-ARQ communication system in which data deterioration is prevented, the number of retransmission is reduced by performing an optimum retransmission requesting process depending on the state of data reception and an error-correction-coding process, and quality and throughput of data transmission with a transmitter are improved.